colloid
/ KOL-oyd /
Think of milk. It looks like a smooth white liquid, but it is actually a swarm of countless tiny fat droplets spread evenly through water — far too small to see, yet large enough to make the milk cloudy and white. A colloid is any material like this: little bits of one substance scattered and floating throughout another.
A colloid is a mixture in which particles of an in-between size — much bigger than single molecules, but still far too small to settle out under gravity — stay suspended and spread out inside a host medium. They are kept afloat by the relentless bumping of the surrounding molecules, the same jiggling that makes them tremble in place. The scattered bits can be solid grains, liquid droplets, or gas bubbles, and the host can be a liquid, a gel, or even a solid.
Colloids matter because so much of daily life is colloidal: milk, paint, fog, ink, blood, and whipped cream are all colloids. A common misconception is that a colloid is just a 'mixture' or a solution. The key difference is size: in a true solution the dissolved bits are individual molecules and the result is clear, whereas a colloid's particles are big enough to scatter light, which is exactly why milk and fog look cloudy rather than transparent.
Shine a flashlight sideways through a glass of milk and you can see the beam light up as a hazy path. The fat droplets are big enough to bounce the light in all directions — a giveaway sign of a colloid that a clear sugar solution would never show.
A visible light beam through milk reveals its scattering particles — the colloid signature.
What keeps a colloid from settling is the same ceaseless molecular bombardment that drives Brownian motion — the particles are jostled around faster than gravity can drag them down.